Field of Invention
The present invention relates to a light emitting device driver circuit and a control circuit and a control method thereof; particularly, it relates to such light emitting device driver circuit wherein a flicker problem is resolved, and a control circuit and a control method thereof.
Description of Related Art
FIG. 1A shows a schematic diagram of a conventional light emitting device power supply circuit 10. As shown in FIG. 1A, the light emitting device power supply circuit 10 converts an AC voltage VAC to an output voltage Vout and supplies an output current Iout to a light emitting diode (LED) circuit 20. The light emitting device power supply circuit 10 includes a TRIAC (TRI-electrode AC switch) dimmer circuit 11, a rectifier and filter circuit 13, and a light emitting device driver circuit 15. The TRIAC dimmer circuit 11 receives the AC voltage VAC (as indicated by a small sinusoidal signal waveform in the figure). When a phase of the AC voltage VAC exceeds a predetermined trigger phase, the TRIAC dimmer circuit 11 fires (i.e. is turned ON) to generate an AC dimmer voltage Vdim (as indicated by a small phase-cut sinusoidal signal waveform in the figure). The rectifier and filter circuit 13 rectifies and filters the AC dimmer voltage Vdim to generate an input voltage Vin (as indicated by a small DC signal waveform with ripples in the figure). FIGS. 1B and 1C show schematic waveforms of the AC voltage and AC dimmer voltages Vdim1 and Vdim2 with different trigger phases, wherein the AC voltage VAC is indicated by a dash line and the AC dimmer voltages Vdim1 and Vdim2 are indicated by solid lines. The rectifier and filter circuit 13 receives the AC dimmer voltage Vdim (e.g. Vdim1 or Vdim2), and rectifies the received AC dimmer voltage Vdim to generate a corresponding input voltage (e.g. Vin1 or Vin2), which is then provided to the light emitting device driver circuit 15. The light emitting device driver circuit 15 is coupled to the rectifier and filter circuit 13, for converting the input voltage Vin to the output voltage Vout, and provides the output current Tout to the LED circuit 20.
In the aforementioned circuit, the function of the TRIAC dimmer circuit 11 is to determine the trigger phase of the AC dimmer voltage Vdim, so as to adjust an average brightness of the LED circuit 20. Different trigger phases of the AC dimmer voltages Vdim1 and Vdim2 can result in different output currents Iout, whereby the brightness of the LED circuit 20 is different. For example, as shown in FIGS. 1B and 1C, wherein FIG. 1B shows the AC dimmer voltage Vdim1 with a lower (earlier) trigger phase, and FIG. 1C shows the AC dimmer voltage Vdim2 with a higher (later) trigger phase, which are rectified and filtered by the rectifier and filter circuit 13 to correspondingly generate a DC input voltage Vin1 with a higher level and a DC input voltage Vin2 with a lower level Vin2, the output current Iout corresponding to the input voltage Vin1 is relatively higher and the output current Iout corresponding to the input voltage Vin2 is relatively lower. As such, by adjusting the trigger phase, the brightness of the LED circuit 20 can be adjusted.
The aforementioned prior art has the following drawback. In the aforementioned prior art, the brightness of the LED circuit 20 is adjusted according to the input voltage Vin. In an ideal condition, the level of the output voltage Vout perfectly corresponds to the level of the input voltage Vin, and the level of the output current Iout perfectly corresponds to the level of the output voltage Vout; however, in a real condition, when the trigger phase exceeds (is later than) a certain angle, or when the TRIAC dimmer circuit 11 turns OFF the LED circuit 20, the leakage of the TRIAC dimmer circuit 11 will cause charges to be accumulated, i.e., the leakage will lead to an increase of the input voltage Vin and therefore an increase of the output voltage Vout, and to a certain extent a non-zero output current Iout is generated and the LED circuit 20 glimmers; after this discharge, the output voltage Vout drops to a lower level, and then increases again because of the leakage. The LED circuit 20 will flicker perceptibly because of the increase and decrease of the output voltage Vout.
In view of the foregoing problem, the present invention provides a light emitting device driver circuit and a control circuit and a control method thereof. The present invention generates a bleeder current in a predetermined trigger phase range by a hysteresis control method, such that the output current is kept zero, and the light emitting device circuit circuit is confirmed OFF, to solve the flicker problem.